Electrophotographic printing is generally known in the art. Typically, a photoconductive material is coated onto a drum or a belt to form a photoconductor. The photoconductor is provided with a uniform electrostatic charge in the absence of light using a suitable charging device. The photoconductor is then exposed to light by an imaging system which imagewise discharges the uniform electrostatic charge to form a latent electrostatic image corresponding to the information to be printed. Imaging systems include scanning laser beams and linear arrays of light-emitting diodes. The latent electrostatic image is developed with a resin powder, called toner to form a visible toner image on the photoconductor. The toner is then transferred to paper and the toner image is fixed to the paper by heating or by the action of solvents. In this way an image is obtained electrophotographically on plain paper.
In recent years, color electrophotographic systems have become commercially available. In the early systems, color printing and/or copying was carried out by laying down upon the paper substrate the different colored portions of the page separately and in succession until the desired full colored print was obtained. For each color, the normal electrophotographic steps were followed. However, the development station was changed in each color cycle so that the appropriate color was transferred to the photoconductor. Thus, each page of color printing required as many cycles as colors desired. This process was tedious and slow and there was a desire by those skilled in the art to decrease the number of cycles.
Gundlach, "Method for Two-color Development of a Xerographic Charge Pattern", U.S. Pat. 4,078,929, issued Mar. 14, 1978, the disclosure of which is incorporated herein by reference, discloses a process for creating multiple charge patterns on a photoconductor and developing the latent images with positive and negative charged toners in one cycle.
May, "Tri-level Highlight Color Printing Apparatus with Cycle-up and Cycle-down Control", U.S. Pat. No. 4,811,046, issued Mar. 7, 1989, the disclosure of which is incorporated herein by reference, discloses color electrophotography utilizing both charge and discharge area development.
Kohyama et al., "High-speed Color Laser Printing Process", Journal of Imaging Technology, Vol. 12, No. 1, Feb. 1986, pp. 47-52, discloses a discharge area development (DAD) method for achieving four color printing in a single cycle. Development is achieved by applying toner to the discharged areas of the photoconductor. The process provides that each layer of toner is retained on the photoconductor until the complete, fully colored image is built up. This is achieved by positioning around the photoconductor four development systems, four imaging systems, and four recharge coronas. After the full color toner image is built up on the photoconductor, it is then transferred to paper. In this case, the printing speed is equal to the black and white electrophotographic process speed.
In a commercially available Panasonic color copier, a different approach has been taken for the color electrophotographic process. As in the Kohyama process, the four color toner images are built up on the photoconductor with DAD development prior to a single transfer to paper. However, in this case the photoconductor rotates four times, once for each color, building up the color images in succession.
For both the Kohyama and Panasonic processes, the photoconductor must be recharged and re-exposed between each development step. It has been discovered that the toner layers adhering to the surface of the recharged photoconductor will cause a reduction in the net change in potential of the underlying photoconductor upon exposure to light. Thus, after the second exposure to light, the discharge potential of the photoconductor will vary depending on whether it has a toner overcoating. This variation in the potential of the discharged portion of the photoconductor will result in uneven deposition of the second and subsequent toners on the photoconductor with the effect of uncontrolled color variations.
It is the object of the present invention to provide a new process for plural color electrophotographic printing.
Other objects and advantages will become apparent from the following disclosure.